1. Field of the Invention
This invention relates to novel filler compositions comprising calcium carbonate, sodium silicate, and organosilane coupling agents, to processes for preparing said filler compositions, and to their use in polymer composites.
2. Description of the Prior Art
It has long been known that various particulate inorganic substances can be used as fillers to improve the economical and/or mechanical properties of natural and synthetic resins. It has been known for almost as long that the benefits of siliceous fillers, such as silica, quartz, asbestos, glass, and various clays on the mechanical properties of polymer composites can be significantly enhanced by employing organosilane coupling agents in addition to the fillers.
Calcium carbonate is a widely available and economical (i.e., cheap) substance which in particulate form would be suitable for use as a filler. However, presumably because it is not siliceous in nature, it is not very responsive to treatment with organosilane coupling agents; without such treatment, polymer composites filled with calcium carbonate do not show satisfactory mechanical properties.
Two approaches have been developed to modify the surfaces of nonresponsive fillers such as calcium carbonate. One deals with the coprecipitation of such fillers with silicas: U.S. Pat. Nos. 2,470,577, 2,865,781, 3,152,001, 3,443,890 and British Pat. No. 838,903. The other deals with in situ precipitation of insoluble silicates over the filler surface: U.S. Pat. Nos. 2,259,481, 2,259,482, 2,259,483, and 4,167,423.